Examples of currently commercialized secondary cells include nickel-cadmium cells, nickel-metal hydride cells, nickel-zinc cells, and lithium secondary cells. Among such various secondary cells, lithium secondary cells are freely rechargeable because of having substantially no memory effect compared with nickel-based secondary cells, and have a very low self-discharge rate and high energy density. Owing to these merits, there has been high interest in lithium secondary cells.
In general, lithium secondary cells use a lithium-based oxide as a positive electrode active material and a carbonaceous material as a negative electrode active material. A lithium secondary cell may include: an electrode assembly in which a positive electrode plate coated with such a positive electrode active material and a negative electrode plate coated with such a negative electrode active material are disposed with a separator therebetween; and a case, that is, a cell case in which the electrode assembly and an electrolytic solution are sealed.
In general, according to case types, lithium secondary cells may be classified into a can type in which an electrode assembly is accommodated in a metal can, and a pouch type in which an electrode assembly is accommodated in a pouch formed of an aluminum laminate sheet.
In recent years, secondary cells have been widely used not only in small-sized devices such as portable electronic devices, but also in medium to large-sized devices such as automobiles or power storage devices. For use in such medium to large-sized devices, a large number of secondary cells may be electrically connected to increase capacity and output power. In particular, pouch-type secondary cells are widely used in medium to large-sized devices owing to merits such as lightness and ease of stacking.
In the related art, when a battery pack is constructed by stacking a plurality of pouch-type secondary cells and a plurality of cartridges to form a cell assembly and accommodating the cell assembly in a pack case, end plates may be provided on both the outermost sides of the cell assembly in the stacking direction of the pouch-type secondary cells. In general, the end plates are formed of a metallic material and have a function of protecting and fixing secondary cells and cartridges and maintaining surface pressure.
In addition, the end plates may be coupled to the cell assembly using fastening members such as bolts. That is, bolts may be inserted through the end plates and the cell assembly, and then both ends of the bolts may be fixed to maintain the coupling between the end plates and the cell assembly. Particular, in many cases, the bolts are inserted into cartridge holes in a state in which ends of the bolts are fixed to an end plate.
In this case, the angle between the end plate and the bolts may have effects on the assemblability and quality of the battery pack. That is, when the angle between the end plate and the bolts is a right angle, the bolts may be easily inserted into the cartridge holes, and a process of assembling the end plate and cartridges and a process of assembling the cartridges may be precisely performed. However, if the angle between the end plate and the bolts is not a right angle, the bolts may not be easily inserted into the cartridge holes.
In particular, when the cartridges are vertically stacked, the cartridge holes may be connected to each other in a direction perpendicular to a surface on which the cartridges are stacked. Therefore, if the angle between end plate and the bolts is not a right angle, when the cartridges are stacked while inserting the bolts into the cartridge holes, the cartridges may not be precisely stacked. In addition, during a process of stacking the cartridges, the angle between the end plate and the bolts may be adjusted to precisely stack the cartridges. In this case, however, the bolts or the end plate may be broken or deformed, and thus the cartridges may not be properly fixed by the bolts.